Constructional elements and method of prestressing same



July 30, 1968 G. M. ADIE 3,394,510

CONSTRUCTIONAL ELEMENTS AND METHOD OF PRESTRESSING SAME Original FiledNov. 23, 1962 2 Sheets-Sheet l July 30, 1968 G. M. ADIE 3,394,510

CONSTRUCTIONAL ELEMENTS AND METHOD OF PRESTRESSING SAME Original FiledNov. 23, 1962 2 Sheets-Sheet 2 United States Patent 3,394,510CONSTRUCTIONAL ELEMENTS AND METHOD OF PRESTRESSING SAME George MountfordAdie, 7 Carlos Place, London, England Continuation of application Ser.No. 239,553, Nov. 23, 1962. This application July 12, 1966, Ser. No.564,626 6 Claims. (Cl. 52223) This application is a continuation of myUS. application 239,553, filed Nov. 23, 1962, now abandoned.

This invention relates to constructional elements for use in buildingsand the like. It is applicable to elements in the form of slabs, planksand the like comprising at least two sheets, at least one of which isformed with a regular series of either dimples or channels the apices ofthe dimples or the bottoms of the channels being secured to an adjacentsheet. Such an element will be hereafter referred to as an element ofthe type described.

The invention is concerned with the deliberate prestressing of such anelement. This is done in accordance with the invention by subjecting atleast one of the sheets of the element to an applied force to stress thesheet either before or after it is secured to the other sheets of theelement. If the stressing is carried out before fastening the sheet tothe element the stress is preferably caused by subjecting the sheet toan applied force in the plane of the sheet and then securing thestressed sheet to an unstressed sheet or oppositely stressed sheet ofthe element by fastening the apices of the dimples or the bottom of thechannels of one of the sheets to the other sheet and then releasing theforce applied to the first sheet. Due to the various points ofconnection between the sheets, the previously stressed sheet or sheetscannot return to their original unstressed condition without distortingthe sheet to which they are fastened. The construction of the sheets ismade such that distortion is controlled and all sheets then becomestressed. If the element consists only of two sheets the first sheetremains stressed in the original sense (tension or compression) but to alower degree, and the second sheet which was previously unstressed, isstressed in the reverse sense.

One method of stressing a preformed element is to cast a layer ofconcrete or other suitable material on to the uppermost sheet of theelement the concrete being preor post-tensioned by means of wires, thestress being transferred to the uppermost sheet and thence to the othersheets.

A further way of stressing the element is to connect bars rods, stripsor the like which are stressed by an applied force either in tension orin compression, at released places, on a side of a preformed element andthen releasing the applied force.

If a dimpled sheet is stressed in tension and then secured to a seconddimpled sheet, the apices of the dimples being fastened together, theapplied force being then released, the first sheet loses half itstension which appears as compression in the second sheet. Such anelement is stronger than an unstressed element. If the element or slabis positioned so that the sheet in tension is uppermost, and the sheetin compression is lowermost, then it will be capable of spanning furtherdistances for a given quantity of material, or of carrying heavier loadsfor a given span compared with a similar unstressed slab. For example,the

3,394,510 Patented July 30, 1968 slab can be 'used as permanentshuttering to support wet concrete over a longer span than an unstressedslab without propping, or alternatively the element can be usedvertically for other load bearing requirements.

If desired the sheets can be stressed in one or more directions forexample two directions at right angles in the plane of the sheet. For arectangular sheet for example stress can be applied in the direction ofboth pairs of opposite sides giving additional strength to slabs whichin use can be supported at all four sides and for a hexagonal sheetstress can be applied at three pairs of opposite sides.

Both the sheets maybe dimpled in which case the sheets may be joinedeither by fastening the apices of the dimples of the two sheets togetheror by securing the apices of the dimples of one sheet to the flat landspresent between the dimples of the other sheet. In the latter case theforces are transferred direct from the dimples of one sheet to the flatlands in the plane of the sheet. In the case where the slab comprises adimpled sheet and a plane sheet either the dimpled sheet or the planesheet may be initially tensioned. For convenience it may be preferableto tension the plane sheet and to fasten an unstressed dimpled sheet toit.

The dimples may have any convenient regular outline such for example ashemispherical, conical, pyramidal or parabolic and they may have anyconvenient cross section-al shape such for example as circular,triangular, square, hexagonal or any other regular polygonal shape. Thedimples are conveniently, but not necessarily, truncated so that thoseon one sheet can be more firmly fastened to those of the other sheet.The dimples of one sheet may be deeper than those of another sheet.

The sheets may be made of any suitable material such for example asmetal, e.g., steel or aluminium, plastic or pressed fibre and the methodused for securing the sheets together will depend upon the material ofthose sheets. For example, if both sheets are of metal or plastic theymay be fastened together by welding or by adhesive, if either or both ofthe sheets are of pressed fibre they may be riveted together.

For some uses it is desirable for one of the sheets to be of onematerial say one particularly suited for being stressed in compression,and the other of another material, say one particularly suited for beingstressed in tension. Furthermore the thickness and quality of the twosheets can be different if required.

The two stressed sheets may comprise part of a composite slab in whichthe initially stressed sheets are secured to other plane or dimpledsheets.

Not all the dimples of the two sheets need to be joined together for allpurposes.

If desired, the element can be stressed when arched giving a convexshape in tension. This form is suitable for some specialised purposessuch as for example a door slab.

In a multisheet slab that side which has been stressed for tension willbe placed uppermost and that side which has been stressed forcompression downwards. Thus, the concrete is placed on the uppermostside and the tension will serve to support the wet concrete which wouldotherwise cause compression of the top sheet. Placed thus the stressedelement can support a great weight of concrete.

It will be realized that alternatively the compressive force can beapplied to the slab if required, whilst it is being made instead of, oras Well as, a tensile force.

Various embodiments of constructional elements and methods ofprestressing the elements in accordance with the invention will now bedescribed with reference to the accompanying drawings in which:

FIGURE 1 is an isometric view of part of a sheet for an element inaccordance with the invention;

FIGURE 2 is a cross-section through one form of slab comprising two ofthe sheets illustrated in FIGURE 1;

FIGURE 3 is a cross section of yet a further form of slab;

FIGURE 4 is a diagram illustrating the method of stressing a slab insitu;

FIGURE 5 is a cross section of a slab having a plane centre sheet;

FIGURE 6 is a fragmentary plan view of a slab stressed by means of rodsapplied to its upper surface, and

FIGURE 7 is a section diagrammatically illustrating a four sheet slab.

The sheet shown in FIGURE 1 has a regular series of conical dimples 2formed therein, the apices 4 of the cones being truncated. The dimplesare arranged on the sheet in parallel rows and on a rectangular grid sothat a series of passageways, indicated by dot and dash lines 66 isformed by the lands 8 between the dimples. These passageways extendacross the sheet and are arranged on a rectangular grid.

One of the sheets shown in FIGURE 1 is gripped along the entire lengthof each of a pair of opposite sides by fiat steel jaws or vises. Thesejaws, which have to be capable of exerting pressure, are then pulled inopposite directions by hydraulic or other means, so as to stress thesheet to any desired figure, say 20,000 pounds. This has the effect ofslightly elongating the length of the dimpled sheet, and if this is sayfrom 10 to feet long the elongation may be of the order of A" dependingupon the material of the sheet. This sheet will then be stressed byforces of up to 20,000 pounds.

A second sheet similar to that shown in FIGURE 1, but which isunstressed, is then placed on the first sheet with the truncated dimplesof the two sheets abutting as can be seen in FIGURE 2, In practice thesetruncated faces may be slightly offset from each other by, in thisexample, a maximum of an Ms, but an adequate overlap is still provided(the figures do not show the overlap, as they only illustrate a veryshort length of the slab). The truncated faces of the pyramidal dimplesof the two sheets are then secured together by welding, or by rivets 10or by other means, whilst the stressed sheet remains under load. Thetotal area of the welds or the number, side and disposition of therivets, is arranged so as to resist the forces which result when thejaws are released and the forces which will be caused later when theelement is subjected to its designed load. The pressure between the jawsis then released and immediately the stressed sheet seeks to return toits former shape and size, but this is resisted through the connectionsto the unstressed sheet. A system of forces is then set up which may beconsidered as being a closed system operating in the slab and thetension in the initially stressed sheet diminishes to the point at whichit is balanced by a corresponding compression in the second sheet.

When the element is positioned so that the sheet in tension isuppermost, and the sheet in compression is lowermost, it is muchstronger than it would he were it to be unstressed and can carry greaterloads and/or span longer distances. This can result in the saving ofsteel or other structural material.

The means for tensioning or compressing the first sheet need not becontinuous along an edge and a number or the like (for tensioning) canbe used, each applying a different force to the sheet so that the stresspattern can be adjusted and the tendency to distort controlled or adesired distortion produced.

In the construction shown in FIGURE 3 the upper sheet, generallyindicated at 14, which is of the same general design as that shown inFIGURE 1, is secured to an identical lower sheet, generally indicated at16 by securing the truncated apices of the conical dimples together. Alayer of concrete 18 is then cast on to the sheet 14, which concrete iseither preor post-stressed by means of wires 20 passing through it. Thestressed concrete subjects the sheet 14 to stress and stress is alsotransferred from the sheet 14 to the sheet 16. Hence the element(including the concrete layer) becomes prestressed.

Stressing may be carried out on a building site by, for example, weldingor otherwise joining a sheet 26 (see FIGURE 4) which is formed withdimples arranged as on the sheet shown in FIGURE 1, but with the dimplesextending upwards to an existing part of a building structure,diagrammatically shown at 24, which may be in the process ofconstruction, and then applying a stressing force to another sheet 26 ofthe slab, and securing the stress sheet, whilst it is subjected to theapplied force, to the first sheet, and also if desired to the buildingstructure. On release of the applied force, the slab will becomestressed in the desired manner. It will be appreciated that this stresscan be achieved by performing the operation in a different sequence.

FIGURE 5 illustrates a construction in which a stressed dimpled sheet 28is secured to an initially unstressed sheet 30 through a centre planesheet 32. Alternatively the plane sheet may be stressed and secured totwo unstressed dimpled sheets and/ or the plane sheet can be arranged atone or both sides of the two dimpled sheets.

FIGURE 7 illustrates a slab produced by a further prestressing methodwherein bars or rods 34 are subjected to an applied force '(eithertensile or compressive) and then secured to the upper face of a dimpledsheet 36, which is the outer sheet of a multisheet slab. On releasingthe ap plied force, the stress in the rods is transmitted via the pointsof connection, to the sheet 36 and to the other sheets of the slab sothat a closed and balanced stress system is set up within the slab.

An element prestressed in accordance with the invention can comprise anydesired number of dimpled sheets fastened either directly together, orwith the interpositioning of plane sheets, FIGURE 8 illustrates a formsheet slab, the upper or two upper sheets being stressed in eithertension or compression and the lower or two lower sheets in compressionor tension respectively.

If dimpled sheets are employed for the slab, the slab will have passagesor paths running through it in two or more directions along the lines 6depending upon the arrangement of the dimples, which passages can beused to carry pipes, wires and the like.

The invention is applicable not only to a method of prestressing a slab,plank or like element, but also to the element when stressed.

I claim:

1. A method of prestressing a constructional element having at least twosheets, at least one of which sheets is formed with a regular series ofdimples, the said method comprising subjecting at least one of thesheets of the element to a tensional force in its plane flat conditionto stress the sheet, securing the stressed sheet to another sheet of theelement and then releasing the tensional force.

2. A method as claimed in claim 1 in which the said one sheet isstressed in more than one direction.

3. A method as claimed in claim 1 in which the element includes a flatsheet.

4. A method as claimed in claim 1 in which all the sheets are dimpledthe apices of the dimples of the said stressed sheet being secured tothe apices of the dimples of the said other sheet of the element.

5. A method of prestressing a constructional element having at least twosheets, at least one of which is formed with regular series of dimples,the said method comprising subjecting at least one of the sheets of theelement to a compressive force in its plane fiat condition to stress 5the sheet before it is secured to the sheets of the element, securing itto the said other sheets, and then releasing the applied force, and inWhich the element includes a flat center sheet which is subjected to anapplied compressive force before and while it is secured to therelatively outer sheet of the element.

6. A prest'ressed constructional element having at least two sheetssecured together, at least one of said sheets being formed with aregular series of dimples, one of the sheets of the element supporting alayer of concrete containing stressed wires.

References Cited UNITED STATES PATENTS Leake 52-225 X Chamberlain 52-573X Kernmer 52-622 X Barnes 52-48 Greulioh 52-223 FRANK L. ABBOTT, PrimaryExaminer. JAMES L. RI'DGILL, JR., Examiner.

6. A PRESTRESSED CONSTRUCTIONAL ELEMENT HAVING AT LEAST TWO SHEETSSECURED TOGETHER, AT LEAST ONE OF SAID SHEETS BEING FORMED WITH AREGULAR SERIES OF DIMPLES, ONE OF THE SHEETS OF THE ELEMENT SUPPORTING ALAYER OF CONCRETE CONTAINING STRESSED WIRES.